The invention relates to a method of setting a position of an object of measurement in layer thickness measurement by X-ray fluorescence according to the precharacterizing clause of claim 1.
Setting the correct position of the object of measurement with respect to the primary X-radiation and with respect to the detector is crucial for the correctness of the measurement when measuring thin layers or multiple layers. For such layer analysis, an X-ray fluorescence radiation of individual elements of a specimen is detected and converted into layer thickness(es) and composition(s). Apparatuses which have an X-ray tube in a housing which is substantially opaque to X-rays are used, with emergence of an X-ray beam being provided through an opening. The extent of the X-radiation is restricted to a specific surface area of the specimen by a collimator. In this surface area, an object of measurement is positioned at a defined distance from the collimator on a table which is movable with respect to the measuring head, comprises an X-ray tube, collimator and the other components required here. The measuring head has, furthermore, a proportional counting tube or a detector, which serves for recording the fluorescence radiation of the irradiated area of the surface.
The distance between the collimator and the surface of the object of measurement has to be set to a specific distance or exact measurement, in order that the fluorescence radiation can be recorded with sufficient intensity.
DE 40 03 757 discloses an adjustment in which the collimator itself is used directly as part of the adjustment. In this case, it is provided that the tip of the collimator is moved against the specimen, with the collimator yielding correspondingly on account of a resilient suspension. Subsequently, further relative movement between the specimen and apparatus is stopped and the device draws the collimator away from the specimen again. The distance between the collimator and the specimen can be set by the amount of spring deflection of the collimator. This apparatus has the disadvantage that damage to the surface can occur. In addition, there are inaccuracies in the setting of the distance on account of production inaccuracies and the paths to be moved along, with a cumulative effect of the errors occurring.
A brochure from the company Veeco Instruments Inc., 1997 edition, likewise discloses an apparatus for measuring thin layers by X-ray fluorescence analysis. In this case it is provided that a beam of an optical recording device is projected into the beam of the X-radiation, in order to be able to view the object of measurement. In the case of this method, a laser beam is used for setting the critical distance for the reproducibility of the measurements. This laser beam falls obliquely on the surface of the object of measurement. During upward and downward movement of the object of measurement, the point of impingement of the laser beam shifts for example from right to left on the surface. Cross wires are superimposed in the recording device and are adjusted to the X-ray beam. As soon as the laser of the laser beam projected onto the surface of the object of measurement coincides with the cross wires, the exact working distance is set. This upward and downward movement of thd object of measurement in relation to the measuring head can be performed manually by an operator, with considerable deviations in the said setting being obtained in these measurement results.
Furthermore, this brochure discloses an automatic laser focusing which is intended to increase the reproducibility of the exact setting. This automatic laser focusing method of setting the measuring distance with respect to a surface of an object of measurement has the disadvantage that the surface impinged is only indistinctly visible in the case of highly reflective surfaces, which leads to an inaccurate height setting. The finite size and unsharpness of the specimen surface impinged by the laser leads to setting errors. Furthermore, an additional laser and corresponding shielding are required.
It is also disadvantageous in the case of both methods mentioned that tilting of the specimen surface cannot be recorded.
The invention is therefore based on the object of providing a method of setting a point of impingement of an X-radiation on an object of measurement defined by a distance of a collimator from the surface of the object of measurement which, on its own, makes an exact setting of this distance possible.
This object is achieved according to the invention by the features of claim 1. The steps provided according to the invention for carrying out the method allow an automatic setting of the surface of an object of measurement at a defined distance from the collimator to take place, with a high degree of accuracy of reproduction being obtained for the position of the surface of the object of measurement with respect to the collimator. In addition, additional sources of error can be eliminated by recording the changes in brightness of the image points of an image, as is the case for example with laser focusing with regard to the point of impingement. Furthermore, the accuracy of reproduction in comparison with manual focusing can also be significantly improved. By evaluating the changes in brightness of the image points during the changing of the distance between the surface of the object of measurement and the collimator, automatic setting can take place without additional apparatus. For this purpose, the electronic recording device, which has a beam projected into the beam of the X-radiation, is used in order that the exact setting of the distance between the collimator and the object of measurement is carried out. By ascertaining the maximum difference in brightness of the image points of the images recorded, a fixed, defined distance of the surface of the object of measurement from the collimator can be set. The beam of the electronic recording device is advantageously adjusted in such a way that the focal point lies in a measuring plane which is at the exact distance from the collimator. When a maximum difference in brightness is ascertained, it can be ensured that a sharp image is recorded by the recording device, and that, as a result, the defined distance has been set. The advantageous assignment of the change in brightness of the image points in a measuring plane to a Z coordinate makes it possible that, after ascertaining a maximum difference in brightness of the image points of an image while moving over the path, an exact setting of the distance can be carried out by positioning the surface of the object of measurement and the collimator with respect to each other.
The object of the invention is similarly achieved by an alternative method according to the features of claim 2. The recording of the differences in brightness of the image points of at least one measuring plane and the ascertainment of the maximum take place in analogy with the method according to claim 1. As a difference from the latter, an assignment of the image in a measuring plane to a Z coordinate is not envisaged. The maximum difference in brightness of the image points of an image is advantageously ascertained and the distance between the surface and the collimator is changed once more, a change in direction being envisaged here. During the changing of this distance, the difference in brightness of the image points of an image in respective measuring planes in turn approaches the maximum. As soon as a comparison establishes that the current maximum coincides with the maximum ascertained when the distance was changed the first time, the changing of the distance is interrupted, whereby a focusing of the image and consequently a defined distance of a collimator from the surface of the object of measurement is set.
According to an advantageous refinement of the invention, it is provided that individual measurements for ascertaining changes in brightness of the image points of an image are carried out during a changing of the distance between the collimator and the surface and the individual measurements are carried out at freely preselectable intervals in time or virtually continuously. As a result, the amount of information to be processed, on the one hand, and the speed of the changes in the absolute amount of a preferably freely settable preselectable path, on the other hand, can be determined.
According to a further advantageous refinement of the invention, it is provided that, for ascertaining the maximum value of the difference in brightness in a measuring plane within a path, the image points y1 to yN are determined in a differential method according to the function F=xcexa3(yixe2x88x92yright neighbour)2+xcexa3(yixe2x88x92yupper neighbour)2, where yi is the brightness value of the image points used. As a result, the difference in brightness between a right neighbour and an upper neighbour can be ascertained, so that the entire information of the image points is recorded when the difference in brightness is formed. This ascertained function value is evaluated for the comparison with other function values ascertained by individual measurements.
According to an advantageous refinement of the invention, it is provided that the changing of the distance between the collimator and the surface of the object of measurement corresponds to a path in which at least the exact distance between the collimator and the surface of the object of measurement is passed through. On account of the advantageous setting of the beam of the electronic recording device, the focal point of which lies in the surface of the object of measurement which corresponds to the exact distance of the collimator from the surface of the object of measurement, it is made possible that a first unsharpness, for example above, and a further unsharpness of the focal point, for example below the surface of the object of measurement, is obtained, whereby the maximum of the difference in brightness lying at the focal point can be ascertained with certainty.
According to a further advantageous refinement of the invention, it is provided that the individual measurement is ascertained from a number of individual images at a time interval, and that an average value is formed from the values of the individual images. As a result, possible disturbing influences such as noise on account of divergent values can be minimized.
According to a further advantageous refinement of the invention, it is provided that the changing of the distance is retained during the individual measurement. As a result, changing takes place without any jerks or jolts, whereby the quality can at the same time be increased for the recording of the changes in brightness of the image points. Furthermore, depending on the time intervals, real-time recording can take place for the individual measurement.
According to a further advantageous refinement of the invention, it is provided that the changing of the distance in the approximate search for a maximum of the differences in brightness is carried out at an increased speed. As a result, an approximate position of the exact distance to be set between the collimator and the surface of the object of measurement can be ascertained in first approximation.
According to a further advantageous refinement of the method, it is provided that, for a precision search, the distance between the collimator and the surface of the object of measurement is reset after carrying out the approximate search to a second starting point at a resetting speed. This resetting speed is advantageously greater than the speed of the approximate search, so that rapid carrying out of the setting is made possible.
According to a further advantageous refinement of the method, it is provided that the precision search is carried out at a reduced speed in comparison with the approximate search. This can make it possible for the individual measurements for ascertaining the function value F to be carried out in much closer steps. After carrying out the precision search, the maximum is ascertained by calculating the zero crossing of the first derivative as an approximation by interpolation. Due to possibly image-typical uncertainties, such as for example noise, several maximums may formally occur, but are prevented by smoothing.
According to a further advantageous refinement of the method according to claim 1, it is provided that, after the precision search, the maximum of the approximate search and of the precision search are compared with each other and a path of movement by which the distance between the collimator and the surface is changed by during the precision search after passing through the maximum is calculated. As a result, after the precision search, a direct setting of the correct distance can be obtained.
According to a further advantageous refinement of the method according to claim 1, it is provided that, before the commencement of the approximate search, a preset distance between the collimator and the surface of the object of measurement is increased by an absolute amount. In this way it can be ensured that, in the subsequent approximate search, a maximum is passed through with a high degree of certainty, it being observed during the increase in the distance whether the difference in brightness decreases. As a result, it can be established at the same time that the starting point for carrying out a measurement lies below the focal point of the exact distance in order to permit a reliable setting thereafter. If the differences in brightness were to increase, the process would be stopped and an indication given to the user that another position is being preselected in order to carry out the setting.
According to a further advantageous refinement of the method according to claims 1 and 2, it is provided that the image points ascertained for recording the difference in brightness in an image are recorded separately in individual fields. This makes it possible for the orientation of the surface of the object of measurement to be ascertained by comparison of the individual fields with one another. The positionally correct orientation is of significance in particular in multiple layer measurements and in the measurement of very thick layers. Recording the orientation of the specimen surface makes it possible to compensate for inaccuracies from an ideal orthogonal orientation of the measuring plane with respect to the X-ray beam.
For this, it is advantageously provided that the value of the maximum change in brightness is recorded in every field. As a result, a comparison between the individual fields can be made possible. If, for example, two fields neighbouring each other have the same change in brightness, it can be concluded from this that this area has no difference in height. If a number of fields have an approximately equal value of a change in brightness, it is ascertained that the planar surface of the object of measurement has a positionally correct orientation, in other words is positioned orthogonally with respect to the X-ray beam.
According to a further advantageous refinement of the invention, it is provided that at least a division into four fields is chosen and, for the characterization of a tilting, the coefficient from a right-hand pair of individual fields and a left-hand pair of individual fields and the coefficient from an upper pair and a lower pair of individual fields are formed. This characterizes the tilting or orientation of the surface of the object of measurement. It is advantageously provided that the sum of the squared coefficients is compared with a constant which is a measure of the orthogonality of the surface with respect to the X-ray beam. Depending on the constant, the tolerance can be pre-formed such that it is greater or smaller.
According to a further advantageous refinement of the invention, it is provided that the differences in brightness within each field are recorded and compared with the neighbouring fields and the orientation is ascertained, a table with an inclination correction being activated in an XY plane with respect to the collimator. This can make possible an adjustment of the orientation of the surface deviating from the ideal plane with respect to the X-ray beam.
Further advantageous embodiments are specified in the further claims.